Ever since the creation of our planet, through Pangea and today, the formation and movement of continents and tectonic plates is a constant process. The movement of the tectonic plates themselves, which float on the “grainy” sphere, and due to the flow of the “grainy” substrate itself, they move, collide and pull on each other. Because of this (when stress occurs because the motion is stuck), earthquakes occur.
Interestingly, one of the earliest instruments for studying earthquakes, constructed by Zhang Heng in 132 AD, was cylindrical in shape like a jug and had eight dragon heads on the top rim. To indicate the direction of the earthquake, he would drop one of the balls from the dragon’s mouth into the mouth of a frog-shaped metal object that sat below each dragon. Knowing the direction was extremely important to send aid to the affected regions on time. The size of the instrument itself, which was 2 m in diameter, is known, while the mechanism itself is unknown.
The sudden development of seismometry and seismology began after devastating earthquakes at the end of the 18th and the beginning of the 19th century. The devastating earthquakes of that period prompted numerous studies, supported and funded by the government, to learn more about the effects of earthquakes and where they occur. The greatest contribution of the scientists of that period was the collection of a large amount of scientific data on earthquakes.
Luigi Palmieri constructed the first electromagnetic seismometer in 1855. The seismometer had U-shaped tubes filled with mercury and oriented in different directions. In the event of an earthquake, the mercury would move, and an electrical contact occurs which stops the clock and starts a registration drum that records the movements of the float that was on the surface of the mercury. This method of measurement made it possible to collect data on the time of the earthquake itself, its relative intensity and its duration.
One of the problems that arise when measuring ground motion is how to maintain a fixed reference point that is not in firm contact with the ground, i.e., remains fixed when the ground moves. In order to obtain a relative displacement in relation to the motion of the ground, inertia was used, that is, the fact that a body at rest wants to remain at rest. This is precisely why, due to greater inertia, the first instruments had large masses. To overcome this problem, measuring devices based on the pendulum principle were constructed. The first seismograph was constructed by the Italian physicist Filippo Cecchi in 1875, using the pendulum principle. Cecchi’s seismograph was the first to record the relative motion of the pendulum with respect to ground motion as a function of time, and it was possible to determine the time of the earthquake’s onset and its duration.
The great development and progress of seismology as a science and profession took place in 1880 after the earthquake in Japan. The consequences of that earthquake were not devastating, but they led to the establishment of the Seismological Society of Japan. The Japanese government has decided to bring foreign experts to the country to develop seismology as a scientific discipline and to provide answers to questions related to earthquakes. One of the most important scientists who stayed in Japan at that time is certainly the English physicist and geologist John Milne, who led the founding of the Seismological Society of Japan. Various measuring instruments were constructed under the auspices of the Seismological Society, while the most important of them is Milne’s seismograph with a horizontal pendulum with levelling, which could detect several types of seismic waves and estimate their speed. Milne’s contribution to seismology also lies in the fact that in 1895 he proposed a worldwide network of instruments. The improvement and development of seismology led to seismology becoming a university department for the first time in 1886 in Japan. The development of seismometry in Japan also influenced numerous European scientists who further improved the now-known seismographs.
In 1886, the German astronomer and geophysicist von Rebeur-Paschwitz constructed a horizontal pendulum with the aim of studying the oscillation of a plumb bob under the influence of astronomical bodies, but it turned out that his pendulums were also sensitive to the horizontal acceleration of the ground. His instruments installed in Potsdam and Wilhelmshaven in 1889 recorded ground shaking caused by an earthquake in Japan. This was the first known record of such a distant earthquake (Figure 3). He was also the first to use photographic paper to record oscillations. The realization that strong earthquakes could be detected over great distances helped usher in the modern era in the field of seismology and the physics of the Earth’s interior.
During the 19th century, scientists such as Cauchy, Poisson, Stokes and Lord Rayleigh came to new knowledge about wave motion, which led to new theoretical predictions about the nature of seismic waves. However, the level of development of seismometry at that time did not allow the confirmation of theoretical predictions. Only after 30 years, in the 20th century, theoretical predictions about the existence of three main types of waves (P, S and surface) were confirmed when they were identified on seismograms.
New seismographs by E. Wiechert in 1904 and Galitzin in 1906 were constructed with measuring devices whose recording was much more detailed than the previous ones and thus attracted the great interest of mathematicians and physicists around the world. The number of seismological stations in the world is growing, the accuracy of the time record was improved by radio signals, seismograms were copied and archived on microfilm, and the data became widely available with the establishment of WWSSN – the first global network of seismological stations. Also, with the help of the new seismographs, numerous new phases were discovered.
Croatian geophysicist Andrija Mohorovičić is responsible for installing two Wiechert seismographs on Grič in Zagreb in 1908. The earthquake near Pokupsko in 1909, recorded on Weichert’s seismogram and many others in Europe, gave him insight into a large database, with the help of whose analysis he tried to explain the way earthquake waves spread through the Earth’s interior. He interpreted the observations of the collision of two seismic waves at some seismological stations as the propagation of the waves along different paths and at different speeds. These findings led him to the conclusion that the Earth’s interior is not homogeneous, but that there are layers of different properties, whereby seismic waves refract and reflect from one layer to another. All these analyses and conclusions proved to him that the discontinuity, i.e., the boundary between the Earth’s crust and the Earth’s mantle, exists. That border area was named Mohorovičić’s discontinuity in his honor. Mohorovičić’s discovery is one of the greatest natural science discoveries in history and changed the way we study earthquakes.
With the development of industry and electrical components, the size and weight of measuring instruments, which used to weigh up to several tons, have been reduced, making them more accessible and easier to transport. The first digital seismographs in the 1980s opened completely new possibilities that are still used today, such as 24-hour data collection, fast data transmission by satellite, digital radio, the Internet, and computers enabling various simulations. Ground shaking data is thus available within seconds of a strong local earthquake. Today’s seismographs are equipped with electromagnetic sensors, where a pendulum with a coil moves in the permanent magnetic field of the housing and thus induces an electric voltage. The electrical signal is further processed and stored in memory. To reconstruct ground motion based on seismograms, seismographs simultaneously record three mutually perpendicular components of ground motion (north-south, east-west, up-down).
Seismographs were also installed on the surface of the Moon in 1969 during the Apollo program with the aim of discovering the internal structure and tectonic activity of the Moon. Similar instruments were placed on Mars in 1976. New research and improvements in technology and the large availability of data have led to advances in the knowledge and understanding of seismic activity on the Moon and Mars. At the end of 2018, NASA landed the InSight robotic probe on Mars with three primary instruments, SEIS, HP3, and RISE, to study the internal structure, and understanding the formation and evolution of the planet through the geological activities of Mars. InSight’s seismograph SEIS, covered by a semicircular dome, records the seismic vibrations of Mars. Just as seismographs provide us insight into the Earth’s internal structure, SEIS gives us insight into the internal activity of Mars. In their new paper published in Nature Communications in October 2022, Croatian scientist Dr Hrvoje Tkalčić and associate Dr Sheng Weng confirmed the existence of the Martian core and determined its size.